Data transmission method, base station equipment and mobile station

ABSTRACT

A base station equipment, mobile station and data transmission method in a digital CDMA cellular radio network, in which base stations communicate with mobile stations located in an area. Each mobile station can have a duplex connection with more than one of the base stations simultaneously. Signals transmitted from more than one of the base stations can be combined in a respective mobile station. A signal received from a mobile station in more than one of the base stations can be combined in a base station controller. In the network, traffic channel transmission is carried out by using a predetermined frame structure. Cellular network frames of different types are transmitted between respective ones of the base stations and the mobile stations over radio paths. To simplify the structure of the base station equipment, the type of frame being transmitted is indicated by a predetermined frame identifier. If the frame identifiers of the frames received simultaneously via several paths are similar, the frames are combined, but if the frame identifiers are different, the frames are not combined.

This application claims benefit of international applicationPCT/FI95/00083, filed Feb. 17, 1995.

BACKGROUND OF THE INVENTION

The present invention relates to a data transmission method in a digitalCDMA cellular radio network, in which base stations communicate with themobile station located in their area, in which a mobile station can havea duplex connection with more than one base station simultaneously; andin which signals transmitted from more than one base station can becombined in a mobile station; and in which a signal received from amobile station in more than one base station can be combined in a basestation controller, and in which network the traffic channeltransmission is carried out by using a predetermined frame structure,and in which cellular network frames of different types are transmittedbetween a base station and mobile stations over the radio path.

CDMA is a multiple access method, which is based on the spread spectrumtechnology and which has been applied recently in cellular radio systemsalong with the earlier deplayed FDMA and TDMA systems. CDMA has severaladvantages over the previous methods, such as spectral efficiency andthe simplicity of frequency planning.

In a typical mobile telephone environment, the signals between a basestation and a mobile station propagate along several paths between thetransmitter and the receiver. This multipath propagation is primarilydue to the reflection of the signal from the surrounding surfaces.Signals that have propagated along different paths arrive at thereceiver at different times due to their different propagation delays.CDMA differs from the conventional FDMA and TDMA in that multipathpropagation can be used in receiving a signal. The receiver generallyused in the CDMA system is a so-called rake receiver, which consists ofone or more rake branches. Each branch is an independent receiver unit,the function of which is to compose and demodulate one received signalcomponent. Each rake branch can be adjusted to synchronize with a signalcomponent propagated along an individual path, and in a conventionalCDMA receiver, the signals of the receiver branches are preferablycombined, a signal of good quality being thus achieved.

The signal components received by the branches of a CDMA receiver may betransmitted from one or more base stations. In the latter case, it is aquestion of so-called macrodiversity, i.e. a form of diversity by whichthe quality of the connection between a mobile station and a basestation can be improved. In CDMA cellular radio networks,macrodiversity, which is also referred to by a term "soft handover", isused for ensuring the operation of power control in the fringe areas ofbase stations and for enabling smooth handovers.

In macrodiversity, a mobile station communicates with two or more basestations simultaneously. The same information is transmitted over allconnections. The base stations do not establish independent signallingconnections with the mobile station, but all signalling connections haveto start from a common switching node. The reliability of the connectionbetween a mobile station and a base station system is based on thecombined reliability of several base station connections.

In the known CDMA systems, the combining of signalling messages and userdata (speech or data signal) on a traffic channel must be similar overall macrodiversity connections. The traffic between different basestations and a mobile station must thus be identical over allconnections. If this were not the case, the mobile station would not beable to combine different signals, and macrodiversity would not bebenefited from at all. Therefore, the multiplexing and demultiplexing ofsignalling and the actual user data must take place in a common locationaccording to the present-day methods.

In the case of a GSM-type network, this is illustrated by FIG. 1. Thefigure shows three base stations BTS1, BTS2 and BTS3, which arecommunicating with a mobile station MS. The signal received from thebase stations is transmitted along lines 11a-11c to a base stationcontroller BSC, in which the signalling information and the actual dataare demultiplexed. The signalling is applied via a signalling unit 10 tothe MSC along a separate line 13, and the speech is transmitted via atranscoder unit TRAU further to the MSC along a separate line 14. Inthis solution, the base stations are merely transmitter/receiver unitswithout signal processing and intelligence. The units provided withintelligence are concentrated within the BTS, wherefore the BSCstructure is complicated and difficult to implement. Lines 12a-12crepresent signalling connections from the BSC to the base stations BTS1,BTS2, BTS3, the BSC being able to give commands to each BTS along theselines. The signalling transmitted via this path cannot, however, beapplied further to the mobile station MS, as in the conventional GSMsystem, but the signalling intended for a mobile station must be appliedvia the TRAU unit in order that it can be multiplexed with the speech ina centralized manner. The CDMA-BSC implemented with the known methodswould thus differ considerably from the base station controllers of thepresent-day GSM network.

SUMMARY OF THE INVENTION

The object of the present invention is thus to obviate the problemsassociated with the solution described above. With the use of thesolution of the invention, it is ,for instance, possible to use GSM-typesignalling between a base station and a base station controller in aGSM-type CDMA network. In a situation where macrodiversity is used, itis also possible to use only one base station for transmittingsignalling information while the other base stations transmit user dataonly. The aim is to provide an arrangement in which the BTS attends tothe multiplexing of speech and signalling, and the coding of the speechcan be located in a more unrestricted manner in the network.

This is achieved with the data transmission method described in theforegoing, background section characterized in that in the frames to betransmitted, the type of frame concerned is indicated by a predeterminedframe identifier, and, if the frame identifiers of the frames receivedsimultaneously via several paths are similar, the frames are preferablycombined, but if the frame identifiers are different, the frames are notcombined.

The invention also relates to a base station equipment in a digital CDMAcellular radio network, this equipment comprising one or more basestation transmitter units and a base station controller, which controlsone or more base station transmitter units, which base station unitscommunicate with the mobile stations located in their area; and in whichnetwork a mobile station can have a duplex connection with more than onebase station transmitter unit simultaneously; and in which networksignals transmitted from more than one base station unit can be combinedin a mobile station; and which base station equipment comprises meansfor combining a signal received from a mobile station in more than onebase station units; and in which network the traffic channeltransmission is carried out by using a predetermined frame structure;and in which network frames of different types are randomly transmittedbetween the base station equipment and mobile stations over the radiopath. The base station equipment of the invention is characterized inthat the base station transmitter units comprise means for indicatingthe type of frame concerned by a predetermined frame identifier in theframe to be transmitted, and that the base station transmitter unitscomprise means for identifying the frame identifier of the frametransmitted from a mobile station.

The invention further relates to a mobile station, intended to be usedin a digital CDMA cellular radio network, in which base stationscommunicate with the mobile stations located in their area, and in whichnetwork a mobile station can have a duplex connection with more than onebase station transmitter unit simultaneously; and which mobile stationcomprises means (34) for combining signals transmitted from more thanone base station; and in which network a signal received from a mobilestation in more than one base stations can be combined in a base stationcontroller; and in which network the traffic channel transmission iscarried out by using a predetermined frame structure; and in whichnetwork frames of different types are randomly transmitted between abase station and mobile stations. The mobile station of the invention ischaracterized in that the mobile station comprises means for indicatingthe type of frame concerned by a predetermined frame identifier in theframe to be transmitted, and means for identifying the frame identifierof the frame transmitted from a base station, and means for combiningthe frames received simultaneously via several different paths if theframe identifier of the frames is the same.

The data transmission method of the invention can thus be applied in adigital cellular network, in which a predetermined frame structure isused on the traffic channel in bit transmission. This type oftransmission based on frame structures is typical of digital datatransmission. In the method of the invention, the frames to betransmitted include a certain bit or symbol sequence, which indicatesthe type of frame concerned. Frames of different types are indicated bydifferent identifiers. Two or more types of these identifiers can beused, depending on how accurately the frames are to be distinguishedfrom one another. In the simplest case, it is possible to use one bit,for instance, to indicate whether the frame contains signallinginformation. A signalling frame can contain conventional call controlmessages or other data, such as short messages.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail withreference to the examples according to the accompanying drawings, inwhich:

FIG. 1 shows the above-described part of a prior art GSM-type networkusing CDMA,

FIG. 2 shows a part of a GSM-type network, which uses CDMA and whichapplies the data transmission method of the invention,

FIG. 3a shows a block diagram of the structure of the receiver side ofthe mobile station of the invention,

FIG. 3b shows a block diagram of the structure of the transmitter sideof the mobile station of the invention,

FIG. 4a illustrates, at the level of the traffic channel framestructure, a situation where the frames which are transmitted from basestations and which contain signalling arrive at a mobile stationsimultaneously,

FIG. 4b illustrates, at the level of the traffic channel framestructure, a situation where the frames which are transmitted from basestations and which contain signalling arrive at a mobile stationnonsimultaneously,

FIGS. 5a and 5b show in more detail a part of a GSM-type network, whichuses CDMA and which applies the data transmission method of theinvention, and FIG. 6 shows a signalling diagram of a system applyingthe method of the invention.

DETAILED DESCRIPTION

In the following, the invention will be described by using as an examplea digital GSM cellular radio system, which has been modified by usingthe CDMA multiple access method on the radio path. However, theinvention is not restricted to the GSM network but it can also beapplied in other types of digital cellular radio networks applying CDMA.It is assumed that the cellular network is synchronized, i.e. that thebase station clocks are in sync with one another.

The GSM base station system presently comprises three units, thefunctions of these units and the interfaces between them being clearlydefined. These units are shown in FIG. 2. The base station unit BTS isassigned the radio path, it multiplexes speech and signalling, andmonitors the quality of radio connections. The base station controllerBSC monitors the use of resources (such as channels) and the signallingrelating thereto. It also manages the communication between the BTSwithin its area. The transcoder unit TRAU attends to transcoding andspeed adjustment for speech and data.

For the sake of example, assume that a message of the third OSI layer istransmitted via the base station controller BSC to the mobile station MSin a situation according to FIG. 2, where macrodiversity is used andwhere the mobile station MS communicates with more than one of the basestations BTS1, BTS2 and BTS3 simultaneously. Each base stationcommunicates with the signalling controller 10 of the base stationcontroller and with the transcoder unit TRAU via a group switch GSW.Each connection between a base station and the base station controlleris independent. The signalling connection between the BSC and a BTS usesthe LAPD protocol. The message of the third OSI layer transmitted by theMSC is transmitted to the base stations along two separate data links.The base stations can receive signalling frames from the signallingcontroller and speech frames from the TRAU at slightly different timeseven if they were transmitted simultaneously. Each base stationmultiplexes the signalling information and the user data by replacing aframe containing user data (for instance speech) by a signalling frame.Since the base stations operate independently, they may place a framecontaining the same signalling information in places differing slightlyfrom each other in the frame structure of the radio path.

The mobile station receives signals from several base stations andcombines them in a preferable manner. Macrodiversity reception can beimplemented, for instance by using the rake receiver structure. FIG. 3ashows a block diagram of the structure of the transmitter side of themobile station of the invention. The receiver comprises an antenna 30,radio frequency parts 31, several rake receiver branches 32a-32c, means33a-33c for indicating the frame identifier, and means 34 for combiningand selecting signals. Each rake receiver branch 32a-32c can besynchronized with a signal propagated along an individual path, thissignal being transmitted by one or more base stations. The combiningmeans can be controlled by means of a control signal 37 according to theupper layer protocol (LAPD-type link layer protocol).

Due to the above-mentioned reasons, the receiver of the mobile stationcan receive different frames from the base stations simultaneously. Thisshould be taken into account in the combining process. This situation isillustrated in FIGS. 4a and 4b. FIG. 4a shows the frames received by themobile station from different base stations. The frame 40a, 40bcontaining signalling information is received in a different frame fromdifferent base stations. Since the frames are different, the informationcontained by them would be lost if the mobile station tried to combinethem as such. In the method of the invention, a frame is provided withan identifier on the basis of which a mobile station can identify thetype of the frame. The indication of the frame identifier must occurbefore the signal-combining block 34. The indication can be carried out,for instance by means provided in connection with each rake branch,these means detecting one of the predetermined bit sequences or symbolsin the frame. Thus, if the combiner 34 of the mobile station observesthat the frames received simultaneously at different branches aredifferent, it will not try to combine them. The combiner of the receiverroutes the frames to destinations in accordance with their frameidentifiers. Speech frames are applied to the speech decoder andsignalling frames to protocol layers, where the termination of the LAPDmprotocol connection is.

The frames containing the same signalling information can thus arrivenonsimultaneously at the receiver. According to a preferred embodimentof the invention, if the frame received first is successfully decoded,the frame which arrives later and which contains the same informationcan be discarded as useless. This takes place on the basis of the linklayer control via the control signal 37. Correspondingly, if thesignalling frame arrived first at the receiver is found to be defective,it can be discarded on the basis of the link layer control when a secondframe arrives.

The combining means 34 can be controlled from an upper link layer alsoin such a manner that priority is given to the frames of a certain basestation. The priority can be based on the received signal strength, forinstance.

Another advantage of the method of the invention is that if a mobilestation receives at different times the frames which contain signallinginformation and which are transmitted by the base stations, thereception of user data is not interrupted because of the signalling,because the signalling frame and the data frame can be processedindependently of each other at the same time. In a normal case, thetransmission of a signalling frame requires the space of one data framein the frame structure.

In a normal case, the frames received by each rake branch at the sametime contain the same frame identifier. This situation is illustrated inFIG. 4b. The frame 41a, 41b containing signalling information isreceived in the same frame from different base stations. The normaldiversity combining is thus possible, and the received frame istransmitted either to the speech decoder or protocol layers.

A frame identifier is similar to the so-called stealing flag used in thefast GSM signalling, but it can be implemented as either a bit, bitsequence, coded or uncoded symbol, depending on how much information isto be transmitted by it.

In the other direction of transmission between the mobile station and abase station, the situation differs slightly from what has beendescribed above. FIG. 3b shows a block diagram of the structure of thetransmitter side of the mobile station of the invention. The transmittercomprises a microphone 60, means 61 for coding speech, means 62 forchannel-coding speech, and means 63 for multiplying speech by aspreading code. After being multiplied by the spreading code, the signalis applied via the radio frequency parts 31 to the antenna 30. Thetransmitter also comprises other components, such as filters andconverters, which are not essential as regards the invention and whichare not included in the figure for the sake of clarity. The transmitterof the invention comprises means 62, in which the frame to betransmitted is provided with an identifier, on the basis of which thereceiver of a base station can identify the type of frame concerned.

The diagrams of the base station equipment of the invention in FIGS. 5aand 5b illustrate the situation on the receiver side in this directionof transmission. In the figures, the mobile station MS transmits to twobase stations BTS1, BTS2, which communicate with the base stationcontroller BSC. In this situation, only one signal is transmitted fromthe mobile station, but it is received in more than one base station.There is thus only one signalling frame on the transmitter side, thissignalling frame being transmitted in the place of a data frame, if needbe. The base stations receive the signal independently, for instance bya rake receiver similar to the one used in the mobile station. The basestation comprises means (50a, 50b) for indicating the frame identifier,on the basis of which it can separate the signalling information fromthe data. The user data is transmitted along a path 52 allocated forthis use on the Abis interface to the base station controller BSC. Asfor the signalling, it is transmitted to the base station controlleralong a normal data link 53 by using the LAPD protocol.

In the base station equipment, the transcoder unit TRAU processes theuser data separately from the signalling. Physically, the TRAU can belocated separately from the base station controller. In the arrangementaccording to FIG. 5a, the TRAU comprises means 51 for combining thesignals of the data frames transmitted from several base stations,whereas the signalling information is processed separately by the secondOSI layer protocol. In the arrangement of FIG. 5b, the means 51 arelocated in the group switch GSW, and otherwise the arrangement issimilar to the one in FIG. 5a. Since two separate data links exist fortwo different base stations BTS for signalling, the received signallingmessages are combined on the basis of the message. For instance, if onesignalling frame is lost over some MS-BTS connection, a correspondingmessage received over another MS-BTS connection can be used.

FIG. 6 illustrates the method of the invention in a GSM-type network bymeans of a signalling diagram. The figure shows the signalling layers ofthe mobile station MS, the base station unit BTS and the base stationcontroller. The traffic between the mobile station and the base stationstakes place on the radio path in the layer L1, and the correspondinglayer between the base stations and the base station controller takesplace via a standardized Abis interface. In the upper level, the frametransmitted in the traffic between the mobile station and the basestation is thus provided with a frame identifier (ID). The trafficbetween the base station and the base station controller takes place bymeans of the LAPD protocol. The termination (LINK LAYER) of the datalink layer between the mobile station and the base station controller islocated in the base station controller after the termination of the LAPDlayer.

In the base station equipment of the invention, the components providedwith intelligence can be located in a more unrestricted manner than hasbeen possible in the prior art. For instance, the TRAU unit can belocated outside the actual BSC. In addition, it is possible to locatespeech coding in a more unrestricted manner than in the solutionaccording to the prior art in FIG. 1. The speech frames arriving fromdifferent base stations can be combined in the TRAU, as described above,or the means 51 for combining can also be located in the group switchGSW, whereby the capacity of the fixed network can be saved when theTRAU is located near the MSC.

Even though the invention has been described above with reference to theexamples according to the accompanying drawings, it is apparent that theinvention is not so restricted but it can be modified in various wayswithin the inventive idea disclosed in the appended claims.

We claims:
 1. A data transmission in a digital CDMA cellular radionetwork in which a plurality of base stations are arranged tocommunicate without a common time reference with a plurality of mobilestations located in an area, in which network a mobile station can havea duplex connection with more than one base station simultaneously, andin which network signals transmitted from more than one of said basestations can be combined in each respective mobile station whichreceives such signals, and in which network a signal received from asaid mobile station in more than one of said base stations can becombined in a base station controller, and in which network trafficchannel transmission is carried out by using a predetermined framestructure, and in which network frames of different types aretransmitted between said base stations and said mobile stations overplural radio paths, said method comprising:indicating for each frame sotransmitted the type of frame by a predetermined frame identifier; andin connection with receiving said transmitted frames, reading therespective frame identifiers and if the frame identifiers of the framesreceived simultaneously via said plural paths are similar, combiningrespective ones of said frames, but if the frame identifiers aredifferent, not combining respective ones of said frames.
 2. The methodaccording to claim 1, wherein:each said frame identifier transmitted andread includes at least one bit allocated as a frame identifier in saidframe structure.
 3. The method according to claim 1, wherein:each saidframe identifier transmitted and read consists of a respective codedsymbol.
 4. The method according to claim 1, wherein:some of said framescontain user data and others of said frames contain signallinginformation of a different type.
 5. The method according to claim 4,further comprising:when a said mobile station in connection withreceiving said transmitted frames from more than one of said basestations and reading such frames applying to a speech coder those of theframes the frame identifiers of which indicate that they are dataframes, and applying to respective protocol layers those of the framesthe frame identifiers of which indicate that they are signalling frames.6. The method according to claim 4, wherein:when a said mobile stationin connection with receiving said transmitted frames receives two ormore respective frames containing the same information from more thanone of said base stations at different times, accepting the first one ofthese respective frames which is successfully received at said basestation, and discarding the respective frames which are received laterand which contain the same information.
 7. The method according to claim5, further comprising:in deciding a respective protocol layer, adecision is made concerning which connections the signalling framesarriving from which to apply to said protocol layers and whichconnections the frames arriving from which to discard.
 8. The methodaccording to claim 4, further comprising:when a said base station inconnection with receiving said transmitted frames from a said mobilestation, said base station separating data frames and signalling framesfrom one another on the basis of said frame identifiers and transmittingthe data and signalling frames separately to said base stationcontroller using respective separate channels.
 9. A base stationequipment in a digital CDMA cellular radio network, comprising:aplurality of base station units having respective transmitters andreceivers; a base station controller, which is arranged to control saidbase station units; said transmitters of said base station units beingarranged to communicate, together with other base station equipment,with which said base station equipment does not have a common timereference, with mobile stations located in an area, each of which canhave a duplex connection with the transmitters of more than one of saidbase station units simultaneously and can combine signals transmitted bythe transmitters from more than one of said base station units; meansfor combining a signal received from a mobile station in the receiversof a plurality of said base station units; said transmitters beingarranged to carry out traffic channel transmission using a predeterminedframe structure; said transmitters of said base station units eachincluding means for indicating for each frame transmitted thereby thetype of frame, by a predetermined frame identifier provided in therespective frame; and said receivers including means for identifying theframe identifiers of frames transmitted from mobile stations andreceived by said receivers, and means for combining the frames only ifthe frame identifiers are the same.
 10. The equipment according to claim9, wherein:said transmitters are arranged to transmit and said receiversare arranged to receive frames of different types respectivelycontaining user data and signalling data of a different type.
 11. Theequipment according to claim 10, wherein:said means for combining asignal received from a mobile station in more than one of said basestation units comprise separate means for combining user data and forcombining signalling information.
 12. The equipment according to claim10, further comprising:a transcoder unit; and wherein:said means forcombining user data received from a mobile station in more than one saidbase station units are located in said transcoder unit.
 13. Theequipment according to claim 9, further comprising:a group switch; andwherein:said means for combining user data received from a mobilestation in more than one of said base station units are located in saidgroup switch.
 14. A mobile station arranged to be used in a digital CDMScellular radio network in which a plurality of base stations without acommon time reference communicate with mobile stations including saidmobile station located in an area, comprising:said mobile station beingarranged to have a duplex connection with transmitters without a commontime reference of more than one of said base stations unitssimultaneously; means for combining signals transmitted from saidtransmitters without a common time reference of more than one of saidbase stations; transmitting and receiving means arranged to carry outtraffic channel transmission by using a predetermined frame structure;means for providing in each frame being transmitted by said transmittingmeans an identifier indicating the type of frame, by a predeterminedframe identifier; means for identifying frame identifiers of respectiveframes transmitted from respective plural ones of said base stations andreceived by said receiving means; and means for combining framesreceived by said receiving means simultaneously via a plurality of pathsif frame identifiers of the respective frames are identified by saididentifying means as being the same.
 15. The mobile station according toclaim 14, wherein:said means for combining are arranged to be controlledfrom upper signalling layers.